A Lithium Dendrite Inhibiting Strategy by Metallic Coatings in Solid Electrolytes via Operando Study

As a next-generation battery technology, solid state batteries, though promising, still face barriers that limit their practical application. Lithium dendritic growth, a main limitation for cell life, is receiving urgent attention within the scientific community because: 1) Li dendrites are easily generated in solid electrolytes and cause early-onset cell shorting and failure; and 2) the understanding of the lithium dendrite mechanism^1,2 remains unsatisfactory. To develop high-performance solid state batteries with long cyclability, inhibiting lithium dendrites is a must. This requires deep knowledge and further study of the dendrite mechanism: when and exactly where Li nucleation occurs, how Li dendrites propagate through solid electrolytes, and what steps will be required to protect solid electrolytes from dendrite penetration. Recognition of this gap in understanding has driven efforts to study the electro-chemo-mechanical effect of dendrites by local characterization which allows immediate observation and direct characterization at the regions of interest.<br/><br/>Here we developed an operando microprobe platform with pressure control, in combination with cryo electron microscopy (Cryo EM) and nanoscale secondary ion mass spectrometry (NanoSIMS), to address this need. We examined a leading solid electrolyte Li_6.6La₃Zr_1.6Ta_0.4O₁₂(LLZTO) of wide interest to the materials community due to its fast ionic conductivity and wide electrochemical stability window³. The highly dense LLZTO sample (&gt;99% relative density) was fractured in an Ar-filled glove box and transferred to SEM by a mechanical air-free transfer cell. The as-fractured surface provides a clean surface without carbonate/hydroxide contamination. A tungsten probe was touched to the as-fractured surface with a controlled contact force, from 0.1 mN to ~5 mN, measured by a well-calibrated spring table. A negative electrical potential was applied between the probe (working electrode) and a counter electrode of lithium metal. We observed that the sample surface under high probe contact force (&gt;2 mN) was cracked by lithium dendrites at a low cell voltage (&lt; 500 mV) while the sample under low probe contact (~0.1 mN) can survive at a higher voltage. The surface nanoscale cracks were shown to be largely responsible for the lithium dendritic deposition.<br/><br/>To inhibit the dendritic deposition, we sputtered the as-fractured surface with a variety of metals including Ag, Pt, and Cu without exposing the LLZTO to air. We then used a focused ion beam (FIB) to cut out microelectrodes between 5 and 20 microns in diameter. We observed that Ag coating of fractured surfaces induced uniform Li plating and high stability at (1) current density &gt; 1000 mA/cm², and (2) cell voltage &gt; 1000 mV at room temperature. In addition, the lithium penetration probability due to lithium dendrites was heavily decreased with only 20 µm² area fractured out of a total searching area of ~2500 µm², in comparison with nearly 100% fracture probability of the uncoated surfaces under the same conditions. The metal-coated surfaces were then examined with Cryo EM and NanoSIMS. The cryo EM analysis showed that the sputtered Ag was able to fill surface nano cracks to a depth of 20-200 nm. The high lithium diffusivity in Ag may circumvent lithium plating at the cracks and avoid hydrostatic stress build-up. In addition, NanoSIMS depth profiling of the isotopes of ¹⁰⁹Ag, ¹³⁹La, ⁹⁰Zr showed that the interface experienced inter-diffusion of cations in compliance with the cryo EELS results which might improve surface energy to prevent dendritic cracks. These insights suggest a strategy of surface defect control and effective coating to inhibit lithium dendrites in solid electrolytes which enables long cyclability of solid state batteries.<br/>Reference:<br/>1. Porz, L. <i>et al.</i> <i>Adv. Energy Mater.</i> <b>7</b>, 1701003 (2017).<br/>2. Han, F. <i>et al</i>. <i>Nat. Energy</i> <b>4</b>, 187–196 (2019).<br/>3. Han, F., Zhu, Y., He, X., Mo, Y. & Wang, C. <i>Adv. Energy Mater.</i> <b>6</b>, 1501590 (2016).